Six volunteer firefighters in Virginia, including two department leaders, accused of allegedly gang-raping a 17-year-old girl — and then posting video of it on social media – have been charged in connection with the case, Virginia State Police confirmed on Tuesday.

Public health officials said it was possible – though unlikely – that clothes or possessions of those who ate and drank in the same restaurant and pub as the Skripals could still be contaminated.

However, a public meeting held at City Hall in Salisbury was told that only the Skripals and DS Nick Bailey had received hospital treatment.

Novichok refers to a group of nerve agents that were developed by the Soviet Union in the 1970s and 1980s to elude international restrictions on chemical weapons. Like other nerve agents, they are organophosphate compounds, but the chemicals used to make them, and their final structures are considered classified in the UK, the US and other countries. By making the agents in secret, from unfamiliar chemicals, the Soviet Union aimed to manufacture the substances without being impeded.

“Much less is known about the novichoks than the other nerve agents,” said Alastair Hay, an environmental toxicologist at the University of Leeds who investigated the use of chemical weapons against Iraqi Kurds in Halabja in 1988. “They are not widely used at all.”

The most potent of the novichok substances are considered to be more lethal than VX, the most deadly of the familiar nerve agents, which include sarin, tabun and soman.

And while the novichok agents work in a similar way, by massively over-stimulating muscles and glands, one chemical weapons expert told the Guardian that the agents do not degrade fast in the environment and have “an additional toxicity”. “That extra toxicity is not well understood, so I understand why people were asked to wash their clothes, even if it was present only in traces,” he said. Treatment for novichok exposure would be the same as for other nerve agents, namely with atropine, diazepam and potentially drugs called oximes.

The chemical structures of novichok agents were made public in 2008 by Vil Mirzayanov, a former Russian scientist living in the US, but the structures have never been publicly confirmed. It is thought that they can be made in different forms, including a dust aerosol that would be easy to disperse.

The novichoks are known as binary agents because they become lethal only after mixing two otherwise harmless components. According to Mirzayanov, they are 10 to 100 times more toxic than the conventional nerve agents.

The fact that so little is known about them may explain why Porton Down scientists took several days to identify the compound used in the attack against Sergei and Yulia Skripal. While laboratories around the world that are used to police chemical weapons incidents have databases of nerve agents, few outside Russia are believed to have full details of the novichok compounds and the chemicals needed to make them.

Jenny Harries, regional director at Public Health England, accepted it was difficult for people to understand why they were allowed to get close to scenes that were being examined by officers in protective hazmat suits.

But she said: “The risk to the general public is low. There are only three cases in hospital. No members of the public have been harmed by this incident. It’s an important message to hang on to.”

Health and council officials, as well as police, promised to be as open as they could to allay fears, and public health representatives will be at the Saturday market in Salisbury to speak to anyone with concerns.

The deputy chief constable of Wiltshire, Paul Mills, said: “46 people have attended [hospital] expressing concern. Each has been assessed but other than the three patients you are aware of no other persons have required hospital admission.

“We have identified 131 people who potentially could have been in contact with the nerve agent and each of these has received calls to ensure their wellbeing. None of these persons have developed symptoms that would indicate they have been exposed to the agent.”

Mills called the nature and scale of the operation “unprecedented”.

He revealed almost 500 police officers and staff were involved backed up by 200 military personnel. There were also 80 ambulance staff on hand every day from nine out of the 10 ambulance trusts nationwide and 50 firefighters. Mills said that cordons could be in place for months to come.

Council leaders accepted that the international reputation of Salisbury could be dented by the attack and the economic impact could be severe.

They announced measures including business rate relief for those affected and the launch of a hardship fund for those worst hit. Park and rides in the city will be free from Saturday until Easter Monday – though there were howls of protest when the council insisted that parking costs in the city would not be reduced.

Asked what the city’s feelings towards Russia, Salisbury’s Conservative MP John Glen said: “People are outraged that a silent assassin could attempt murder. But our message to the people of Russia s that they are always welcome in Salisbury. Our issue is with the Putin regime.”

White House on lockdown after shots fired

“Secret Service personnel are responding to reports of a person who allegedly suffered a self-inflicted gun shot wound along the north fence line of @WhiteHouse,” the Secret Service tweeted.

President Donald Trump and first lady Melania Trump are at Mar-a-Lago in Florida.

“We are aware of the incident,” deputy press secretary Hogan Gidley said. “The President has been briefed. I refer you to the Secret Service for any more information.”

The Secret Service tweeted an update on the situation moments later: “No other reported injuries related to the incident at @WhiteHouse.”

Secret Service spokeswoman Cathy Milhoan said the incident occurred on the north fence line, the victim suffered a single gunshot wound and Secret Service personnel did not fire any shots. She would not comment on the status of the victim.

The agency also tweeted that pedestrian and vehicular traffic around the White House has been affected by the incident.